Extrusion encompasses a plurality of well-known processes that feature low tooling and labor costs, making extrusion a desirable machining process especially for tubular profiles. During an extrusion process, a solid polymer (i.e., pellets, chips, beads, powder and the like) is generally fed through a transport section into a rotating screw or extrusion pipe via a feeder or hopper. Plastic is slowly heated as it is pressed forward toward a die, becoming a homogeneous melt that is subsequently forced through the die to form a continuous-length shape. Once cooled, the extrusion is cut and shaped to a desired length and configuration.
Ram extrusion is a specific extrusion method that utilizes pressure sintering for the continuous production of profiles from high-molecular-weight polymers. A ram forces a pre-formed mass of polymeric material, such as pre-compressed powder, into a die (typically heated) having the requisite cross-section. The individual particles of the mass sinter together under the effects of heat and pressure to generate an extrudate.
Achievement of a desirably thin tube is impaired due to the limitations of conventional extrusion processes. It is desirable to maintain uniform wall thickness during an extrusion process and provide generous radii that improve the flow of the composition through the die and reduce stresses associated therewith, thereby significantly reducing the cost and improving the consistency of the product. For instance, maintaining the uniformity of the wall thickness helps to maintain the maximum running speed of the extruder and the straightness of the part. Because extruded parts are only contained by the metal at the die, they tend to move or sag downline to the detriment of product uniformity. Although it is relatively easy to manipulate the physical and mechanical properties of the material to match performance specifications, it remains nonetheless difficult to manipulate thin-walled members due to the flexible nature of the resultant extruded product.
Additionally, a common problem encountered during extrusion is deviation of tubular thickness and center deviation during the extrusion process. In a ram extrusion apparatus, the die not only shapes the final extrudate, but it also determines the quality of the extrudate by the combined effects of pressure and temperature. Generally, extrusion dies for hollow profiles consist of an extrusion pipe, a mandrel and a ram. In the extrusion of hollow profiles such as tubes, dense powder must slide over two surfaces (i.e., those of the extrusion pipe and the mandrel), thereby inducing escalated pressure during the extrusion of prior art tubes (having relatively thin walls in the range of 2-3 mm thickness) and counter pressure due to friction. Extrusion pressures are thus higher for such relatively thin prior art profiles than those experienced for solid profile extrusion or tubular profiles with increased wall thickness. As a result, when extruding materials, there is an inherent risk of exceeding the upper pressure limit of the powder and getting tablet formation, especially during cooling when the extrudate shrinks onto the mandrel. In addition, the critical components of extrusion equipment (e.g., mandrel; extrusion pipe; die) are subjected to high forces and pressure which cause relative movement between the components. Such relative movement typically results in some off-center, eccentric alignment which is acceptable when extruding relatively thick-walled tubular members. However, with thin-walled structures, the eccentric off-set may cause insufficient material to be directed to a portion or portions of the resulting extrudate. Accordingly, wall portions may result which are unacceptably too thin and/or include discontinuities. Thus, continuous maintenance of the alignment of key extruding elements is critical in forming thin-walled members.
A beneficial material that is widely used in extrusion applications, including ram extrusion, is polytetrafluoroethylene (PTFE). PTFE undergoes cold flow so as to change dimension when pressure is applied, even at room temperature. PTFE can be extruded from a resin powder mixed with a liquid lubricant or from a conventional paste. When properly processed, PTFE exhibits superior properties inherent in fluoropolymer resins, such as non-aging characteristics, chemical inertness, heat resistance, toughness and flexibility, low coefficient of friction, negligible moisture absorption, improved resistance to deformation under load and minimal sagging when in billet form. Beyond PTFE, most polymeric materials are suitable for extrusion, including but not limited to high density and low density polyethylene, rigid and flexible polyvinylchloride, polypropylene, polyurethane, thermoplastic rubber, acrylic, cellulose acetate butyrate, polycarbonate, polyethersulfone and polyphenylene sulfide.
One technique for forming PTFE tubing is to use a wet flow paste extrusion process. The paste extrusion process yields a “green tube” that is then subjected to secondary operations such as heating, expansion and sintering to yield an expanded PTFE (ePTFE) tube having a porous node and fibril structure. The green tube produced by the prior art paste extrusion process has a relatively thick wall and a small internal diameter, and the resultant ePTFE tube exhibits a relatively thick wall thickness. It is desired to obtain structurally-sound thin-walled tubes having thinner walls than that found in the prior art. It is especially desired to obtain thin-walled PTFE green tubes and ePTFE tubes which can be used in medical applications, such as with grafts and stent/grafts.
It is therefore desirable to provide a mechanism and associated method for repeatedly and predictably producing an elongate tubular member having a very thin wall thickness. Such a member is conducive to employment in a variety of applications wherein prolonged patency of the tube complements inherent circumferential strength without compromising a need for minimal wall thickness.